1. Field of the Invention
The invention relates to the field of industrial battery cases. More particularly, the invention relates to a method and apparatus for making a one-piece battery jar having at least a thicker section at the opening thereof than at other areas of the jar.
2. Prior Art
Lead-acid batteries are used in industry to power vehicles such as fork-lifts. Since a single lead-acid cell has a potential of 2 volts, the higher voltages suitable for motors are attained by packing a number of cells upright, side-by-side, in a steel tray, and connecting them electrically in series.
For the larger batteries, each 2-volt cell is enclosed in its own container which consists of a "jar" whose upper end is initially open to allow insertion of the battery-plate assembly, and a cover, which is welded onto the top of the jar after the battery-plate assembly has been inserted.
To achieve its potential life, the plates of a motive-power lead-acid battery must be pressed tightly against each other to minimize the shedding of plate material. For this reason, the outside of a jar is preferably a rectangular prism. This shape enables the cells to be tightly packed in a rectangular tray made of a material that will maintain the packing pressure during the life of the battery. Typically, the trays are made of steel.
The inside walls of the jar which press against the plates, need to provide the same pressure at the top and the bottom of the plates. This requires the jar to have the same inner dimensions at its top and its bottom. If the jar is molded with "draft" where the top is wider than the bottom, the plates may be pressed tightly together at the bottom, but not tightly packed at the top. The less than optimum pressure between the plates at the top results in relatively more shedding of material and shorter battery life.
Other properties of a battery jar that are necessary to produce a battery of high quality are:
1. Leak-proofness PA1 2. Adequate mechanical strength for the life of the battery-plate assembly at the temperatures occurring in the battery. PA1 3. Provision for good heat welding of the cover to the jar. For this requirement, the following conditions need to be met:
A. Sufficient wall thickness all around the top of the jar. PA2 B. Hot-plate welding compatibility between the jar material and the cover material. PA2 C. The jar and cover materials must not stick to the hot-plate welding platens.
To be acceptable in the market, the above qualities have to be provided at the lowest cost. This requires that a minimum of material be used, that lower cost reprocessed material be used if possible, and that the manufacturing process be relatively low cost.
Battery jars have long been made by injection-molding them in one-piece with some degree of draft from polypropylene co-polymer material. The material is injected at the bottom of the jar and flows toward the open end. Typically, wall thickness is 0.120 inches or greater. This results in a jar which uses a relatively large amount of material. It is difficult to reduce the wall thickness because the plastic material flow-length to passage thickness ratio even at a wall thickness of 0.120" is, for a "high" jar, 29 in/0.120 in=241. This is a high value for injection molding and for polypropylene co-polymer materials having melt-indices low enough so that they do not stick to the hot-plate welding platens. It results in very high injection and mold pressures.
To reduce the amount of material needed to form the jar while retaining the strength thereof, the inventor hereof created a profiled wall battery jar. The jar employs thicker material at the ends of the elongated jar and thinner material at the midsection thereof. This achieves strength yet reduces the amount of material needed. Moreover, and importantly, this provides more space for acid while maintaining strict adherence to the outside dimensions dictated by the conventional mounting areas in machines employing the finished batteries. A more detailed disclosure and explanation of this battery jar having a combination of thick and thin walls is found in U.S. Pat. No. 4,732,826 which is assigned to the assignee hereof and incorporated herein by reference. Conventionally, profiled wall battery jars have been molded in half sections and then welded at the seams. The welding is generally of the hot plate type and is in two steps: first to join the two halves and second, to create a three-layer structure in the joint. The operation is particularly suited to providing a leak-free joint but is relatively labor intensive which, of course, increases cost of manufacture. It has been desired to reduce the labor associated with producing profiled battery jars while retaining the benefits thereof Heretofore it was not known how such benefits could all be achieved.